Polyethylene glycol.
Polyethylene glycol ("PEG"), (.alpha.-Hydro-.omega.-hydroxypoly (oxy-1,2-ethanediyl)), is known by numerous designations including macrogol; PEG; Carbowax; Jeffox; Nycoline; Pluracol E; Poly-G; Polyglycol E; and Solbase. PEG refers to the liquid and solid polymers of the general formula H(OCH.sub.2 --CH.sub.2).sub.n OH, where n is greater or equal to 4. In general, each PEG is followed by a number which corresponds to its average MW. Its synthesis is described for instance in Hibbert (1939) J. Am. Chem. Soc. 61:1905-1910. For review see Powell, III in Handbook of Water-Soluble Gums & Resins, R. L. Davidson ed. (McGraw-Hill, New York, 1980) pp. 18/1-18/31. PEGs have found use as water-soluble lubricants for rubber molds, textile fibers, and metal-forming operations, in food and food packaging, in hair preparations and in cosmetics in general and as ointment and suppository bases in pharmaceutical compositions.
Typically, PEGs are clear, viscous liquids or white solids that dissolve in water to form transparent solutions. They are soluble in many organic solvents and readily soluble in aromatic hydrocarbons. They are only slightly soluble in aliphatic hydrocarbons. Typically, they do not hydrolyze on storage. PEGs have low toxicity. PEG20M consists of two or more molecules of PEG having approximate molecular weights of 6000-10,000 joined by a bisphenol epoxide linker (CAS # 37225-26-6; CAS name Oxirane, 2,2'[(1-methyl-ethylidene)bis(4,1-phenyleneoxy methylene)]bis-, polymer with .alpha.-hydro-.omega.-hydroxypoly(oxy-1,2-ethanediyl). PEG20L is a substantially linear PEG having an average molecular weight of about 20,000 Daltons (available from several commercial sources including, but not limited to, Clariant/Hoechst Celanese, Fluka and Nippon Oils and Fats). The molecular weights of PEG compositions listed herein are given in number averages rather than weight averages. Various other molecular weights of linear PEG are also available from several commercial sources.
More recently, PEG has been used in a number of pharmacologic applications. The conjugation of PEG to foreign proteins, such as cytokines and antibodies, reduces the immune response triggered when the proteins are administered into test mammals. U.S. Pat. Nos. 5,447,722; 4,902,502; 5,089,261; 5,595,732; 5,559,213; and 4,732,863. Conjugation to PEG also increases the solubility and biological half-life of cytokines. WO 8700056 and U.S. Pat. No. 5,089,261. Conjugates of PEG and glucocerebroside have been formulated for treating Gaucher's disease. WO 9413311. PEG has also been conjugated to such enzymes as adenosine deaminase, amidase bovine and asparaginase, for therapeutic use. See Delgado et al. (1992) Crit. Rev. Ther. Drug. Carrier Syst. 9:249-304; and Burniam (1994) Am. J. Hosp. Pharm. 51:210-218, for review.
Organ preservation solutions.
Transplantation of vital organs such as the heart, liver, kidney, pancreas, and lung has become increasingly successful and sophisticated in recent years. Because mammalian organs progressively lose their ability to function during storage, even at low temperatures, transplant operations need to be performed expeditiously after organ procurement so as to minimize the period of time that the organ is without supportive blood flow.
In clinical practice, the two major situations in which cardiac preservation is required are heart transplantation and cardioplegia for open heart surgery. In heart transplantation, the donor heart is exposed through a midline sternotomy. After opening the pericardium, the superior and inferior vena cavae and the ascending aorta are isolated. The venous inflow is then occluded, the aorta is cross clamped, and approximately 1 liter of cold cardioplegic solution is flushed into the aortic root under pressure through a needle. As a result, the heart is immediately arrested, and cooling is supplemented by surrounding it with iced saline. The cold, arrested heart is then surgically excised, immersed in cold cardioplegic solution, packed in ice and rushed to the recipient.
The recipient's chest is opened through a midline sternotomy, and after placing the patient on cardiopulmonary bypass, the diseased heart is excised. The preserved donor heart is then removed from the preservation apparatus, trimmed appropriately and sewn to the stumps of the great vessels and the two atria in the recipient chest. After completion of the vascular anastomoses, blood is allowed to return to the heart. The transplanted heart will then either resume beating spontaneously or will require chemical and electrical treatment to restore normal rhythm. When the heart is ready to take over the circulation, the cardiopulmonary bypass is discontinued and the recipient's chest closed.
Most non-transplant surgical procedures on the heart, such as coronary artery bypass grafting, require that the heart beat be arrested for a period ranging from approximately 30 minutes to 120 minutes. The elective stopping of cardiac activity temporarily by injection of chemicals, selective hypothermia or electrical stimuli is termed "cardioplegia." During the period of arrest, the heart is kept cool by external cooling as well as by periodically reflushing a cardioplegic solution through the coronary arteries. The composition of the cardioplegia solution is designed to rapidly arrest the heart and to keep it in good condition during the period of standstill so that it will resume normal function when the procedure is finished.
In these cardioplegic procedures, the heart is exposed in the chest, and at a minimum, the aortic root is isolated. A vascular clamp is applied across the aorta and approximately 1 liter of cold cardioplegic solution is flushed into the aortic root through a needle. Venting is provided through the left ventricle, pulmonary artery or the right atrium and the effluent cardioplegic solution, which may contain high levels of potassium, is sucked out of the chest. This, together with external cooling, produces rapid cessation of contractions. During the period of arrest, the patient's circulation is maintained artificially using cardiopulmonary bypass. If the level of potassium in the cardioplegic solution is sufficiently high, bi-caval cannulation can be used to prevent the build up of potassium in the body that could interfere with later attempts to restart contractions in the heart.
After completion of the surgical procedure, blood flow is restored to the coronary circulation and heartbeat either returns spontaneously or after chemical and electric treatment. The ease with which stable function is restored depends to a large extent on the effectiveness of preservation by the cardioplegic solution. Once the heart is beating satisfactorily, cardiopulmonary bypass is discontinued and the chest eventually closed. General methods for organ transplant and heart surgery are disclosed in D. K. C. Cooper (editor), The Transplantation and Replacement of Thoracic Organs, Boston, Kluwer Academic Publishers (1997); and Collins et al. Kidney International 42:S-197-S-202 (1992) and the art cited therein, and are commonly known in the art.
It is generally understood that "living" organs, including the heart, continue the process of metabolism after removal from the donor so that cell constituents are continuously metabolized to waste products. If the organ storage technique is inadequate, the accumulation of these metabolic waste products, depletion of cell nutrients and consequent change in cell composition lead to progressive loss of function and ultimately to cell death. That is, the organ will lose its ability to function adequately after transplantation into the recipient. Several procedures have been explored to successfully enable organ preservation ex vivo for useful time periods. In one method, the donor organ is cooled rapidly by flushing cold solutions through the organ's vascular system and maintaining the organ at temperatures near 0.degree. C. for the purpose of greatly slowing the metabolic rate. In the case of the mammalian heart, the flush solution composition is designed to quickly cause the heart to stop beating as well as to preserve it.
In 1988, University of Wisconsin (UW) solution was described. Belzer et al. Transplantation 45:673-676 (1988). This solution, capable of preserving the pancreas and kidney for 72 hours, and the liver for 30 hours, subsequently became the standard organ preservation solution (OPS) for transplant surgery and the benchmark against which other OPS compositions were measured. However, the heart is more recalcitrant to long-term storage than other organs, and UW solution is unreliable for storage of hearts for as short a period as 24 hours. Wicomb et al. Transplantation 47:733-734 (1989).
Improvements in the design of OPS compositions, as reviewed in Collins et al. Kidney, International 42:S-197-S-202 (1992) and others described in the art, have proceeded along several paths, including: (1) modification and simplification of UW solution; (2) investigation of organ-specific requirements; (3) addition of pharmacologic agents, particularly calcium antagonists for control of acidosis; (5) the use of a terminal rinse solution; and (6) the use of solutions containing polyethylene glycol (PEG).
Wicomb et al. reported the beneficial effects of PEG 8000 on rabbit hearts preserved by oxygenated low pressure perfusion for 24 hours; this solution also contained horseradish peroxidase. Wicomb et al. Transplantation Proceedings 21:1366-1368 (1989). The substitution of PEG20M for hydroxyethyl starch (HES) as the colloid in UW solution also yielded excellent cardiac function. The substitution of PEG20M for HES also allowed rabbit heart storage up to 24 hours by microperfusion. Wicomb et al. Transplantation 48:6-9 (1989).
An improved OPS, "PEG20M-Cardiosol.TM." used as a heart preservation solution, contained the substitution of PEG20M for HES and eliminated five components of UW solution (penicillin, dexamethasone, insulin, allopurinol, and adenosine). Wicomb et al. Transplantation 49:261-264 (1990); and U.S. Pat. No. 4,938,961, issued Jul. 3, 1990 to Collins et al. That heart preservation solution contains 5% or 10% by weight polyethylene glycol 20M (Union Carbide Chemicals and Plastics Co., Inc., Charleston, W. Va.), 40 mM sodium, 125 mM potassium, 5 mM magnesium, 25 mM phosphate, 5 mM sulfate, 100 mM lactobionate, 30 mM raffinose, and 3 mM glutathione. Collins et al. The Lancet 338:890-891 (1991). This solution was found to be superior to UW solution both for 4-hour hypothermic and 24-hour microperfusion storage. Collins et al. (1992). "Microperfusion" is a hypoxie, very-low-flow perfusion with flowrates such as 3 ml/g heart wt/24 hour, which is 1/500 of that typical of conventional continuous perfusion. Wicomb et al. Transplantation 48:6-9 (1989).
The superiority of PEG in preservation solutions remains widely accepted. Mack et al. Cryobiology 28:1 (1991); Malhotra et al. Transplantation 52:1004 (1991); and Schmid et al. Transplantation 52:20 (1991). A solution containing polyethylene glycol 20 L has been studied for its effect on rejection of transplanted small bowel allografts. Wicomb et al. J. Heart Lung Transplantation 13:891-894 (1994). While the solution was apparently active as an immunosuppressant in small bowel transplant, prolonged survival was not related to improved preservation and no correlation was seen between recovery and the type of flush solution used. Wicomb et al. J. Heart Lung Transplantation 647 (1994).
The mechanism of PEG activity in OPS in unknown. However, as reviewed in Collins et al. (1992), PEG is known to improve tissue viability, reduce ischemic injury by preventing cell swelling, interact with lipids in the cell membrane, and scavenge free radicals. OPS containing PEG20M also appears to blunt the immune response to a transplanted organ previously flushed with a PEG solution. Itasaka et al. Transplantation 57:645-648 (1994); and Tokunaga et al. Transplantation 54:756-758 (1992). PEG20M has also been proposed to be effective when used only as a terminal rinse solution after a period of ice storage. Collins et al. (1992).
There is a need for an improved composition for use in OPS's and other aspects of preserving or restoring the function of cells, tissues or organs that displays improved storage stability under practical commercial conditions.
All references cited herein are hereby incorporated by reference in their entirety.